Nanosecond fast electrically tunable fabry-perot filter

Abstract: The present invention is directed at an electrically tunable filter for wavelength filtering of light. The filter comprises a substrate region (7), a waveguide region (1) over the substrate, an upper region (10) over the waveguide region, and current blocking regions (2) adjacent to the waveguide region. The waveguide region comprises a semiconductor (5) having a bandgap wavelength sufficiently different from the light for transparent transmission of the light. A waveguide rib layer (6) may also be included in the waveguide region to channel the light through the filter. The current blocking regions narrow the waveguide region to increase the current density. The present invention is also directed at a method of manufacturing the filter described above.

Description: A key component of Wavelength Division Multiplexed (WDM) networks is a tunable broadband wavelength demultiplexer. The ideal wavelength filter would be widely tunable with constant transmittance over the entire free spectral range, would be quickly tunable with narrow channel spectrum selectivity (high finesse), and would be integrable with other lightwave components such as amplifiers or detectors. To that end, many different devices have been developed as demultiplexers, such as a piezoelectrically tunable fiber Fabry-Perot filter and a laser diode operated below threshold. A piezoelectrically tunable Fabry-Perot fiber is described in J. Stone and L. W. Stulz, "Pigtailed High-Finesse Tunable Fibre Fabry-Perot Interferometers With Large, Medium and Small Free Spectral Ranges," Elect. Lett. , Vol. 23, pp. 781-783 (1987). This article describes three Fabry-Perot filter prototypes and their test results. All three designs used standard PZT piezoelectric components for tuning. These filters yielded finesse values up to 200. Furthermore, an insertion loss as low as 1.5 dB was observed for lower finesse values. Thus, mechanically adjusted filters have excellent wavelength selectivity and low insertion loss. Since they are mechanically tuned, however, they can only attain tuning speeds in the order of milliseconds. Therefore, a need exists for faster tuning filters. Semiconductor devices with inherently faster tuning speed have been reported by a number of groups. For example, T. Numai et al., "1.5 µm Tunable Wavelength Filter with Wide Tuning Range and High Constant Gain Using a Phase-controlled Distributed Feedback Laser Diode," Appl. Phys. Lett., Vol. 53, No. 13 (Sept. 26, 1988), and Katsuaki Magari et al., "Optical Signal Selection with a Constant Gain and a Gain Bandwidth by a Multielectrode Distributed Feedback Laser Amplifier" Appl. Phys. Lett., Vol. 51, No. 24 (Dec. 14 1987) describe laser diodes with wide tuning ranges and high tuning speed. However, laser filters suffer from gain instability. Gain instability arises in laser diodes because they absorb light at the wavelengths typically used in multiplexing, e.g., 1.55µm. More specifically, the valence electrons of the waveguide region semiconductor absorb the multiplexed light waves and rise to an excited state. Light is therefore absorbed rather than transmitted by a laser diode. To overcome this problem and achieve a "transparent" condition, current is applied across the laser diode. This excites the electrons so that they cannot absorb the light energy. Although this creates a transparent condition, the diode becomes highly excited. In fact, the diode is biased just below the threshold current of the laser. Because the bias hovers just below the threshold current, a slight energy change may cause the diode to cross the threshold and become a laser emitter. The transmission of light, for example, may provide the requisite energy for the diode to cross the laser threshold. Additionally, because tuning requires current injection, the tuning process may push the diode over the laser threshold. If the diode crosses the threshold and becomes an emitter, significant damage may result. In addition to instability, a laser diode suffers from tuning shortcomings. For example, the shape of the transmission spectrum varies as the current varies. Furthermore, current variations produce non-linear shifts in the transmission light wavelengths. Both of these shortcomings hinder the tunability of a laser diode filter. Thus, the need remains for a wavelength filter with not only quick tunability, but also gain stability, constant filter shape, and linear tuning. The present invention fulfills this need. According to one aspect of this invention there is provided an electrically tunable filter as claimed in claim 1. According to another aspect of this invention there is provided a method as claimed in claim 8. The present invention is directed at filtering light of multiple wavelengths. A filter embodying the invention may remain transparent at zero current bias and at all tuning currents for a particular transmission wavelength. This allows the filter to avoid the instability problems inherent with laser diodes. A filter embodying the invention may have quick tunability. Because the present invention is electrically controlled, it can be tuned in nanoseconds. This presents a marked improvement over piezoelectrically tunable Fabry-Perot filters which tune in the order of milliseconds. A filter embodying the invention may have high tunability. This entails a filter with a wide spectral range, a low insertion loss, and a high finesse. Brief Description of the Drawing For a fuller understanding of the nature and object of the invention, the reader should take note of the detailed description taken in connection with the accompanying drawings in which: Fig. 1 shows the structure of a preferred embodiment; Fig. 2 shows four stages of making the filter using one preferred method of forming the current blocking regions; and Fig. 3 also shows four stages of making the filter but uses another preferred method of forming the current blocking regions. Detailed Description of the Present Invention The present invention is a semiconductor wavelength filter with quick tunability, wide spectral range, high finesse, low waveguide loss, and transparent transmittance. A schematic of this device is shown in Fig. 1. The present invention comprises a waveguide region 1, a substrate region 7, an upper region 10, and current blocking regions 2. In the preferred embodiment of Fig. 1, a dotted line demarcates waveguide region 1 from upper region 10. Waveguide layer 1 comprises a waveguide layer 5 and a waveguide rib layer 6. Both these layers consist of a second semiconductor which may be selected from either Group III-V compounds or Group II-VI compounds. The second semiconductor is selected such that it does not absorb light at the transmitted wavelength. The embodiment of Fig. 1 uses Indium Gallium Arsenide Phosphide which ranges in bandgap from 1.2µm to 1.48µm so that it remains transparent at zero bias and at all tuning currents for transmitted light. Waveguide rib layer 6 channels the light down the center of waveguide layer 5 and away from the sides of the waveguide layer 5. This prevents the light from scattering along the rough, etched sides of waveguide layer 5. Waveguide rib layer 6 should be sized to provide a cross-sectional area just sufficient for the light to propagate in its fundamental mode. Substrate region 7 comprises a substrate layer 8 and a first contact layer 9. Substrate layer 8 consists of a first semiconductor which may be selected from either Group III-V compounds, Group II-VI compounds, or Group IV elements. In this particular embodiment, first semiconductor is an n-type Indium Phosphide. First contact layer 9 consists of an n-type contact material. First contact layer 9 is adapted for electrical contact. Upper region 10 comprises an upper layer 11, a transitional layer 12, and a second contact layer 13. Upper layer 11 consists of a third semiconductor which may be selected from either Group III-V compounds, Group II-VI compounds, or Group IV elements. In this embodiment, third semiconductor is a p-type Indium Phosphide doped with Zinc. Second contact layer 13 consists of a p-type conductive material. Like first contact layer 9, second contact layer 13 is adapted for electrical contact, thereby completing a circuit across the filter. Current flow is aided by transitional layer 12 which provides an interface between second contact layer 13 and upper layer 11. In Fig. 1's embodiment, transitional layer 12 consists of Indium Gallium Arsenide doped with Zinc. To increase the current density through waveguide region 1, current blocking regions 2 narrow the current flow through waveguide region 1. Current blocking regions 2 may consist of an insulating material such as Indium Phosphide doped with Iron, or alternatively, may comprise an reversed p-n junction to prevent current flow. The device is tuned by applying current across the filter which injects carriers in waveguide region 1. These carriers cause a change in the refractive index of waveguide region 1. The dependence of the refractive index on the carrier density (dn/dN) was evaluated for the embodiment of Fig. 1 and found to be -6x10 -21 /cm 3 , while the temperature dependence of the index (dn/dT) is 3x10 - 4 /°C. This facilitates tuning ranges exceeding 5.9ηm (740 GHz). It will be apparent to those skilled in the art that the conductivity type of substrate region 7 and upper region 10 can be reversed. That is, rather than substrate region 7 and upper region 10 being a p-type and an n-type conductivity type respectively, they can be an n-type and a p-type respectively. Additionally, while a planar geometry has been specifically described above, a mesa geometry may be preferred in some instances. Furthermore, other substrate orientations are contemplated. The filter is manufactured by first growing a wafer, and then cleaving it into filter chips. The wafer is grown by a combination of Low Pressure (LP) and Atmospheric Pressure (AP) Metal Organic Vapor Phase Epitaxy (MOVPE). The growth and processing sequence is shown in Fig. 2 wherein like parts are like numbered. First, a waveguide layer 21 ranging in thickness from 1000Å to 5000Å is grown over a substrate layer 20. A stop etch layer 22, which can range in thickness from 100Å to 300Å, is then grown over waveguide layer 21. Next, a waveguide rib layer 23a ranging in thickness from 200Å to 500Å is grown over stop etch layer 22. A mask is then placed over the center portion of waveguide rib layer 23a and an etching agent is applied. The agent etches the sides of waveguide rib 23a to form waveguide rib 23b. The width of waveguide rib 23b should not exceed 2µm. Once the waveguide region of the filter is prepared, either alternative step A or B can be performed on the wafer to complete the growth stage. Under alternative A, a thick (1.5µm) upper layer 24c is grown over the waveguide region, followed by a transitional layer 25. Next, a wider mask (8-12µm) is centered over waveguide rib 23b, and deep (25µm) cavities 26 are etched through upper layer 24d into waveguide layer 21d by a combination of reactive ion etching and chemical etching. Since these etched sidewalls are well away from the light channel (i.e., the region under wide waveguide rib 23b), the guided light does not scatter at etching and regrowth imperfections, resulting in very low loss waveguides. In the final growth step, thick (2.5µm) current blocking regions 27 are grown in cavities 26 to confine the current and planarize the structure. Fig. 3 refers to alternative B. Under this alternative, a wider mask (8-12µm) is centered over waveguide rib 33c, and cavities 36 are etched into waveguide layer 31c by a combination of reactive ion etching and chemical etching. Next, current blocking regions 37 are grown in cavities 36 to confine the current and planarize the structure. The last growth step involves growing a thick (1.5µm) upper layer 34 over the waveguide region, followed by a transitional layer 35. Following the growth step, the wafers are metallized, and cleaved into 20-1000µm long filters. Last, thin filmed mirrors are deposited on the cleaved facets by standard deposition techniques. The filters would then be mounted on copper heat-sinks for use.

Claims: An electrically tunable filter for filtering light of a predetermined range of wavelength such that said light remains transparent for at least one particular predetermined wavelength, said filter being further capable of tuning between said received wavelengths at a tuning speed on the order of nanoseconds, said filter comprising: a) a substrate (7) comprising a first semiconductor having a uniform doping concentration; b) a waveguide region (1) formed on said substrate, said waveguide region comprising a second semiconductor having a uniform doping concentration, and a bandgap wavelength sufficiently shorter than said particular predetermined wavelength; c) an upper region (10) formed over said waveguide region, said upper region comprising a third semiconductor; d) current blocking regions (2) adjacent to said waveguide region, said current blocking regions comprising a current blocking material, said waveguide region having narrower width than said substrate region thereby increasing current density through said waveguide region; and e) electrode means (9, 13) for applying current to said waveguide region to cause a change in said waveguide region's refractive index to tune said filter. A filter as claimed in claim 1, wherein said first and third semiconductors are selected from the group consisting of Group III-V compounds, Group II-VI compounds, and Group IV elements, and wherein said second semiconductor is selected from the group consisting of Group III-V compounds and Group II-VI compounds. A filter as claimed in claim 2, wherein said first semiconductor is an n-type material and said third semiconductor is a ptype material. A filter as claimed in claim 2, wherein said waveguide region comprises Indium Gallium Arsenide Phosphide. A filter as claimed in claim 4, wherein said first semiconductor is an n-type material and said third semiconductor is a p-type material. A filter as claimed in claim 5, wherein said substrate region comprises a substrate layer (8) and a first contact layer (9), said upper region comprises an upper layer (11), a transition layer (12), and a second contact layer (13), and said electrode means comprises said first and second contact layers. A filter as claimed in claim 6, wherein said substrate layer comprises Indium Phosphide, said upper layer comprises Indium Phosphide doped with Zinc, and said transitional layer comprises Indium Gallium Arsenide. A method of manufacturing an electrically tunable filter as defined in claim 1, said method comprising: a) growing a substrate layer (20); b) growing a waveguide region (21) over said substrate layer using a combination of Low Pressure (LP) and Atmospheric Pressure (AP) Metal Organic Vapor Phase Epitaxy (MOVPE); c) growing an upper region (24c) over said waveguide region; d) growing current blocking regions (27) adjacent to said waveguide region to increase current density in waveguide region; e) metallizing under said substrate layer and over said upper region to permit current to said waveguide region to cause a change in said waveguide region's refractive index to tune said filter; f) cleaving said layers and said regions to form cleaved facets defining lengths of said filter ranging from 20 to 1000 microns; and g) coating said cleaved facets to form mirror surfaces. A method as claimed in claim 8, wherein said step of growing said waveguide region comprises: (i) growing a waveguide layer (21); (ii) growing a stop etch layer (22) over said waveguide layer; (iii) growing a waveguide rib (23a) over said top etch layer; (iv) placing a mask over the center of said waveguide rib layer; and (v) etching said waveguide rib layer. A method as claimed in claim 9, wherein said step of placing said mask over said waveguide rib layer comprises using a mask so that said waveguide rib is just wide enough to permit fundamental mode wave propagation. A method as claimed in claim 8, wherein said step of growing said current blocking regions comprises: (i) etching the sides of said waveguide region to form cavities (36); and (ii) growing said current blocking regions (37) in said cavities. A method as claimed in claim 8, wherein said step of growing said current blocking regions comprises: (i) etching the sides of said upper region and said waveguide region to form cavities (26); and - (ii) growing said current blocking regions (27) in said cavities.

Inventors: DENTAI ANDREW G, STONE JULIAN, DENTAI, ANDREW G., STONE, JULIAN
Applicants: AT & T CORP, AT&T CORP.
Application year: 1994
Main classification: G02F1
IPC codes: H01S5, G02F1, H01L33